A cellular radio communication system, such as illustrated in the block diagram of FIG. 1 of the accompanying drawings, generally consists of an array of base transceiver stations 12 that are deployed over the area of coverage of the system and connected via a network of broadband links 14 to each other and to a central switch or controller 16. Each base station 12 sends radio signals (over a downlink 17) to, and receives radio signals (over an uplink 19) from, mobile transceiver units 18 (also to be referred to as subscriber units or subscribers) located within an area surrounding it, or within a sector thereof, termed a cell. The mobile transceiver units 18 may be mobile telephones, mobile data communication units or portable or vehicle mounted radio units or the like. The size of a cell (e.g. its radius about the base station) is defined as that over which a usable signal is received from the respective base station and largely depends on the power at which its transmitter operates downlink (to be referred to, for brevity, as the “power” of the base station); this subject is discussed further below. The exact siting and programmed power of each base station are planned, according to, inter alia, the topography of the area, so as to provide continuous coverage by the cells, with overlapping regions. Each base station is assigned a set of physical channels, over which it communicates with the mobile subscriber units within its cell. Such a set of channels is usually characterized by a group of unique carrier frequencies, but may also be distinguished by other multiplexing methods known in the art, such as CDMA (code division multiple access). There is a finite number of such channel sets and these are assigned to the various base stations so that, at the very least, no adjacent cells share a channel set and preferably so that cells or base stations sharing a channel set, termed co-channel cells or co-channel base stations, are mutually distant sufficiently to keep mutual interference below some given threshold value. An ideal layout (which may arise in a situation of homogenous topography and requirements) is schematically depicted in FIG. 2, as an illustrative example. Here the base stations and their associated cells are arranged in a regular hexagonal cell grid and they are assigned seven sets of channels. A channel re-use pattern in a network of cells in a system operating as shown in FIG. 1 is thus shown in FIG. 2. The numerals in the cells in FIG. 2 indicate use in each cell of a particular channel, e.g. frequency, in the set of seven channels numbered 1 to 7. In a practical network, the base stations are generally deployed in a much less regular manner and the shapes and sizes of their associated cells will vary. The present invention is equally applicable to any system, having any cell size and shape and any deployment topology.
Generally each mobile unit can receive signals from several proximate base stations. Each received signal is characterized by a carrier-to-noise ratio (C/N) and a carrier-to-interference ratio (C/I), where the interference is mainly from nearby co-channel base stations. Both ratios depend on the current downlink power of the corresponding base station; the C/I, however also depends on the current power of the nearby co-channel base stations. These ratios can be measured by the mobile subscriber unit and compared against each other, as well as with stored threshold values. A mobile unit is generally programmed initially to scan the channels (e.g. the frequencies) and to select that with the highest C/I and/or highest C/N and then to establish communication with the corresponding base station, which thus becomes the so-called active base station. The mobile subscriber unit is then said to be linked to the active base station. When any of the two ratios drops below a certain respective threshold level (owing to changing location or other causes), in some types of systems the power of the base station at the respective channel is raised and if it has reached its maximum programmed value, which in other systems (notably those based on TDMA—time divison multiple access) is always the case, a so-called reselection process is initiated, whereby the mobile unit selects from among other received signals the one with the highest C/I and/or C/N, switches communication to the corresponding channel and thus becomes linked to the corresponding base station, which becomes its active base station. In effect, the mobile subscriber unit thus moves from one cell to another (usually adjacent) cell. If all mobile units have the same threshold values stored, they would generally undergo reselection, upon moving away from the active base station, at about the same distance—which defines the effective range (or just “range”) of the base station or the boundary (and hence, size) of the cell. Thus, clearly, the effective boundary of any cell is a function of the stored threshold values, as well as of the power of the active base station and of any interfering co-channel base stations for each channel. For any system, threshold values and (as mentioned above) also the base stations deployment topology and power values are generally chosen so that the areas within the effective boundaries of adjacent cells (i.e. the ranges of their corresponding base stations) overlap substantially, in order to have sufficient margin of safety for contiguous service.
It is to be noted that the total power transmitted by any base station is the sum of the powers of all active channels and is limited by the capacity of the transmitter's power amplifier. In general, the power levels of all channels (or their maximum values, in systems where the levels vary according to reception conditions) in any one base station are set so that the total power is below the amplifier's capacity—to correspond with the designed range of the respective cell. This setting of power levels (whether of individual channels or of the overall power) is usually achieved through appropriate programming and thus can, in principle, be changed—e.g. when various conditions change; such a change is, however, not a common practice and is generally done only through intentional directed programming, as part of a system re-design.
Typically, a mobile unit has a programmable digital controller, which stores various parameters related to the communication process, including, in particular, those related to monitoring and calculating C/N and C/I ratios of received signals, identifying adjacent cells and controlling the reselection process. These parameters can be downloaded from the system over a control channel during system operation.
A communication path between any two mobile units 18 within a cellular system usually consists of the radio link between each of them and its respective base transceiver station 12 and the path through the wide-band network 14 between each of the base stations 12. Each radio link between a base station 12 and a mobile unit 18 consists of the downlink 17 and the uplink 19. If the two mobile units are within the same cell, the link may consist of their radio links only, if the base station is so programmed, in which case they are said to be directly connected. One method for such direct connection, for example, is the use of so-called site trunking protocols. Another method, to be explained in the next following paragraph, is aimed at group calls.
Each base station 12 typically includes a digital controller 11 and a digital program store 13 (for software programs and data), which together serve to control the communication process between the linked mobile units 18 and other base stations and the central switch 16 (and where appropriate with agents external to the system). Similarly, the central switch 16 includes a digital program store 15, to serve in controlling communication with all of the base stations 12 and with the external world beyond the system network.
The present invention is applicable to any type of cellular communication system, but it is particularly suitable for use in the so-called Private Mobile Radio system (PMR) or Trunked Mobile Radio (TMR) system—especially when having provision for group calls. In such a system, which generally serves for dispatch mode of service (i.e. ready communication within a “fleet” of mobile subscriber units, usually belonging to one organization), the users share common channel resources, and are directed to use these resources under the control of a central controlling entity; the latter is usually the central switch of the system, but in cases such as addressed by the present invention it may be a fallback base station. During a group call, a defined group of mobile subscriber units (usually a subset of the fleet) is interconnected so that all their operators can listen simultaneously to any one of them talling. In many cases, mobile subscriber units defined to belong to a group are frequently located within a relatively small geographic area (for example, members of a police unit patrolling a town). Often this area is, to a great extent, congruent with a cell—sometimes by design of the system. A group call between mobile subscriber units linked to the same base station is generally carried out by direct connection (i.e. not through the central switch)—usually by means of a shared channel; this is a semi-duplex mode of communication, whereby all members of the group within the cell use the same channel pair, one member doing the talking over the uplink channel and all others listening over the downlink channel. Communication with group members in other cells is carried out over the usual path, which includes the central switch, where the appropriate connections are made.
For normal operation of the cellular network 14 shown in FIG. 1, each base station 12 is connected either by radio communication and/or by a physical line connection to other terminals, especially other base stations 12. Each base station 12 is thereby connected directly or indirectly to the central switch 16 as indicated in FIG. 1. In such normal use, network control signals to and from the central switch as well as traffic signal (signals representing speech, data etc) are sent via this network of links.
Among the faults that may accidentally occur in an operational system, two give rise to problems which are solved by the present invention. The first one occurs when a base station loses its link to one or more other base stations and becomes partially or totally isolated. In consequence, the base station may also become disconnected from the central controller or switch. In such a case, all mobile units currently served by this base station may continue to be served by this station or, if appropriate, may communicate with other mobile units within range directly. However, indirect connections to mobile units served by other base stations normally connected to the isolated base station via the lost link(s) will be lost.
The second operational fault may occur when all, or a large number of, base stations of the network become isolated from one another or from the network infrastructure, for example owing to a fault in the network or in the central switch. In this case, of course, all inter-cell communications will be lost and, again, only intra-cell or direct communications will be possible.
In many installed PMR or TMR systems, large groups of subscribers, with their respective mobile subscriber units, are often located within relatively small geographic regions (e.g. towns), with most of the communication traffic occurring within any group. Much of this traffic may be in the form of group calls. Such a region is typically covered by a few contiguous cells and each mobile subscriber unit is serviced by the base station providing the best signal. If one of these base stations becomes isolated, a so-called fallback situation is declared and a fallback procedure is initiated. The main objective now becomes to keep as many subscribers as possible interconnected—which, in most cases, means keeping as many groups as possible intra-connected. One of two general situations may be discerned: (a) one or more groups are largely located within the isolated cell or immediately adjacent to it; (b) most subscribers within the isolated cell are not interconnected and do not belong to a common group.
There is thus a clear need for a method and system that will further increase the number of mobile units that remain interconnected in fallback situations caused by individual or massive isolation of base stations from the cellular system.
GB-A-280570A describes a procedure for dealing with the isolation of a cell caused by failure of its base station whereby service to mobile units within the cell is lost, i.e. it is assumed that the output from the affected base station is lost. This procedure involves expanding the range of adjacent cells by increase of their output power. The procedure is intended to provide communication coverage to some of the mobile units which have lost service from the failed base station. By implication, the cells adjacent to the one which has failed are on a different frequency to provide this bordering coverage.